how to validate a form field based on the value of the other?

Question

I have 2 form fields,I want to validate the second form field to match the password from the first one,I tried but no success..thanks for answering.

Updated : I already have submit button and its working,I want Validator in the second field to validate the first field text to match the second field.

            new TextFormField(
          controller: _registerPassController,
          decoration: new InputDecoration(labelText: 'Password'),
          obscureText: true,
          validator: (value) =>
              value.isEmpty ? 'Password can\'t be empty' : null,
          onSaved: (value) => _password = value,
        ),
      ],
    ),
    new Stack(
      alignment: const Alignment(1.0, 1.0),
      children: <Widget>[
        new TextFormField(
          controller: _registerPassController2,
          decoration: new InputDecoration(labelText: 'Retype Password'),
          obscureText: true,
          validator: (value) {
            if (value.isEmpty) {
              return 'Please enter some text';
            }
          },),

Answer 1

I finally find the answer,its so simple actually.

        new TextFormField(
          controller: _registerPassController2,
          decoration: new InputDecoration(labelText: 'Retype Password'),
          obscureText: true,
          validator: (value) {
            if (value != _registerPassController.text) {
              return 'Password is not matching';
            }
          },
        ),

Answer 2

Since you are using formfield it will be appropriate to use the key to access the value of other field. You can declare the global key like this

var passKey = GlobalKey<FormFieldState>();

Then pass it to the password field to retrieve its value during validation.

TextFormField(
  key: passKey,
  obscureText: true,
  decoration: InputDecoration(
      labelText: "Password"
  ),
  validator: (password){
    var result = password.length < 4 ? "Password should have at least 4 characters" : null;
    return result;
  },
);

Then you can use the password inside the confirmation validator like this

TextFormField(
  obscureText: true,
  decoration: InputDecoration(
      labelText: "Confirm Password"
  ),
  validator: (confirmation){
    var password = passKey.currentState.value;
    return equals(confirmation, password) ? null : "Confirm Password should match password";
  },
);